1. Field of the Invention
This invention relates generally to a method and composition for forming doped A-site deficient manganate thin films on substrates, and more particularly to a method and composition for making thin film manganate layers via metalorganic chemical vapor deposition (MOCVD) using a liquid delivery technique for flash vaporization of the precursor chemistry.
2. Description of the Related Art
Current magnetic sensors and read/write heads are fabricated from permalloy and multilayer giant magnetoresistant (GMR) metal structures. Permalloy (Ni0.80Fe0.20) exhibits a 2% change in resistivity in small magnetic fields, while the GMR multilayer FeMn/Co/Cu/NiFe exhibits a 4% change in a 10–20 Oe field. The observations of large resistance changes in LaxAyMnO3 (A=Ca, Sr, Ba) crystals in the presence of magnetic fields has generated intense interest in using these materials as magnetic sensors. Thin films of these colossal magnetoresistant (CMR) oxides have exhibited resistivity changes of over 100,000% in magnetic fields of >1 Tesla. Since CMR materials display far larger magnetoresistive changes than GMR materials, they have the potential to greatly improve both the detection sensitivity and signal-to-noise ratios in a MR device. The most suitable material for commercial applications would have a large magnetoresistance change in a small magnetic field at ambient temperatures.
High quality, doped lanthanum manganate films possess numerous properties of technological importance, including temperature sensitive resistance changes, bolometers, substantial magneto-resistance changes under exposure to magnetic fields, and utility for device applications such as switches, sensors, thin-film recording heads and magnetic random access memories (MRAM). The doping of the LaMnO3 system with Group II elements (i.e., Mg, Ca, Sr and Ba) is critical to providing robust materials with the desired electrical, magnetic and thermal responses. In this material system, precise and repeatable compositional control is required in order to produce films of high quality. Physical deposition methods (e.g., sputtering, evaporation) to fabricate thin film deposition are deficient in this regard, as are traditional approaches to metalorganic chemical vapor deposition (MOCVD) involving the use of bubblers.
A great need exists to miniaturize discrete magnetic components. At present, thin film-based devices are an attractive solution to many magnetic sensor and MRAM applications, but require cost-effective processing of films in the thickness range of from about 0.1 to about 10 microns, as well as room temperature electrical and magnetic responses, and large magnetoresistive responses to small magnetic fields.
A number of processes have been investigated to date for depositing manganate films, including pulsed laser ablation deposition (PLD), sputter deposition, and sol-gel processing (SGP). While high quality films have been produced in some cases, no technique has yet been demonstrated which has clear potential for commercially viable device manufacture.
The high rate deposition techniques, which have been used successfully for multicomponent oxide thin films are pulsed laser ablation deposition (PLD) and chemical vapor deposition (CVD). The use of PLD for Ca doped LaMnO3 (LCMO) has been investigated and the quality of films produced was acceptable. However, as the deposition area is scaled in PLD, one loses the inherent deposition rate advantage. Area scaling is important in making manganate layers because, although device and chip dimensions are small, cost-effective deposition in an integrated process is ideally accomplished at wafer scale dimensions. Further, magnetic sensors and random access memories require thin-film application over complex topographical features (i.e., in multi-layered devices).
Another technique, termed sol gel processing (SGP), has been investigated by various researchers. This technique has the advantage of processing at lower temperatures, but unfortunately the resultant film properties have not been as good as desired. In particular, film defects are high and conformality is poor.
It would therefore be an advance in the art, and is accordingly an object of the invention, to provide a method for the formation of thin film A site deficient, doped manganate materials by MOCVD which affords repeatable and stringent stoichiometric control of the manganate film composition, and resulting thin-film properties. It is the stoichiometric composition and the method to deposit the same that constitutes this invention. In specific, A site deficient doped manganates exhibit room temperature electrical and magnetic transitions towards small magnetic fields.
Other objects and advantages of the invention will be more fully apparent from the ensuing disclosure and appended claims.